1. Field of the Invention
The present invention concerns an optical displacement sensor. The present invention is suitably applicable particularly to encoders for measuring a displacement of an object, velocity sensors, acceleration sensors, and length measuring apparatus, utilizing such an effect that when a light beam projected toward a moving object is diffracted or scattered, the diffracted or scattered beam is subject to phase modulation according to a displacement or a moving speed of the object.
2. Related Background Art
There are conventionally used apparatus which are arranged in such a manner that light is projected toward an object and that interference between light beams diffracted or scattered therefrom is utilized to obtain a physical amount such as movement or displacement of the object with high accuracy; for example, optical encoders, laser Doppler velocimeters, laser interferometers, etc.
Features of these apparatus utilizing the light are high accuracy and high resolution in the wavelength-of-light order, but in order to be applied in wider fields, they are required to add a size reduction (the size of millimeter order); stability, "handleability," and durability of an interference optical system; and an origin detecting function.
The present applicant disclosed the optical encoder shown in FIG. 1 as an example having an optical system which is simple, stable, easy to handle, and suitable for miniaturization among high-accuracy optical linear encoders utilizing the interference between diffracted light. In detail, a divergent beam R emitted from a light-emitting device 1 such as LED is converted into a linearly converging beam R' by an anamorphic optical element 2 (for example, a toric lens), the converging beam is thereafter split into two beams R1, R2 by a first diffraction grating G1 on a transparent substrate 4, the beams are then linearly converged to impinge on points P1, P2 on a diffraction grating G2 on a relatively moving scale 20, two beams R1 (+) (+ first-order diffracted light from the point P1) and R2 (-) (- first-order diffracted light from the point P2) reflection-diffracted there are arranged to intersect with each other on a diffraction grating G3 provided on a same plane as the diffraction grating G1, and the beam R2 (-) travels straight while the beam R1 (+) is diffracted in the- first order, so as to superimpose wavefronts thereof on each other to interfere with each other, thus emerging therefrom. The diffraction grating G3 is composed of gratings G3a, G3b, G3c, G3d of same pitches and phases .pi.K/4 shifted to each other, and beams multiplexed by the respective gratings is detected by photodetectors PD1, PD2, PD3, PD4. The photodetectors output respective sinusoidal signals with phases .pi./4 shifted to each other with a displacement of the diffraction grating G2, and an amount of relative displacement of the scale 20 is measured by processing the signals in a signal processing unit PS by a well-known method.
Employment of this optical system achieves an encoder which is easy to handle while keeping the high accuracy, and which is stable in interference state even with a relative positional deviation (e.g., an angular shift such as azimuth, swing or tilt, etc.) between "the diffraction grating G2 on the scale" and "the detecting head portion consisting of the diffraction gratings G1, G3, the light-projection/reception devices, etc.".
There, however, occurs a need to fully control changes of ambient temperature and changes of quantity of emitted light due to degradation with time in order to employ LED excellent in anti-surge characteristics as a light source. Also, there has been a demand to add the origin detecting function to the encoder while keeping the size thereof compact.